Graduate Thesis Or Dissertation
 

Water quality of runoff from flood irrigated pasture in the Klamath Basin, Oregon

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/t722hd40t

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  • Runoff from agricultural lands into Upper Klamath Basin rivers and lakes can cause water quality problems affecting fish and wildlife. Excessive eutrophication in Upper Klamath Lake is linked to high nutrient input (particularly phosphorus) stemming from both lake sediments and watershed tributaries. On a unit area load basis the Wood River Valley contributes a much greater load of phosphorus to Upper Klamath Lake than other regions. The purpose of this study was to measure the export of nutrients from flood irrigated cattle pasture in the Wood River Valley to; 1) compare irrigation water quality with background sources, 2) determine the sources and transport mechanisms of sediment and nutrients on flood irrigated pastures and 3) consider opportunities for water quality improvement. Subsurface and surface water quality samples and water flow measurements were taken on two flood irrigated and grazed pastures. Limited samples were also collected at two additional pasture sites and during a storm. Water samples were analyzed for concentrations of sediment, dissolved organic carbon (DOC), total dissolved nitrogen (TDN), total dissolved phosphorus (TDP), orthophosphate (OP), ammonia (NH₄⁺-N) and nitrate (NO₃⁻-N). Nutrient and sediment loads were calculated and flow weighted concentrations were compared. A nutrient and sediment budget was estimated during two irrigations. On a 70 ha study plot, the TDP and TDN concentrations were highest when cattle were causing disturbance in actively flowing irrigation ditches which had mean values of 0.50 mg TDP L⁻¹ and 2.55 mg TDN L⁻¹. When cattle were not present the dissolved nutrient concentrations in tailwater ditches were lower with mean values of 0.07 mg TDP L⁻¹ and 0.85 mg TDN L⁻¹. An irrigation headwater canal had a seasonal flow weighted concentration (FWC) of 0.03 mg TDP L⁻¹ and 0.22 mg TDN L⁻¹ while seasonal FWC for tailwater ditches was 0.84 mg TDN L⁻¹ and 0.06 mg TDP L⁻¹. On a 2 ha study plot, irrigation tailwater samples were collected directly from pasture runoff. DOC, TDN, TDP and OP concentrations were 2.8, 6.3, 2.8 and 3.2 times greater, respectively, in tailwater than in headwater. A first flush was evident as greatest dissolved nutrient concentrations occurred early in the irrigation runoff period. The mean net export for two irrigations was 24 kg sediment ha⁻¹, 7.4 kg DOC ha⁻¹, 0.43 kg TDN ha⁻¹ and net accumulation of 0.02 kg TDP ha⁻¹. TDN and DOC export progressively decreased during runoff but showed significant net export throughout the entire irrigation. TDP was more variable with an export rate of 0.2 kg TDP ha⁻¹ during the first flush followed by insignificant export and accumulation later during the irrigation runoff period. Most dissolved-N in surface water was in the organic form, NH₄⁺-N was low and NO₃⁻-N was undetected. Sediment concentration in irrigation runoff did not follow the first flush pattern and was dependent on original background levels or on cattle disturbance. Shallow groundwater had low mean concentrations of TDP (<0.06 mg TDP L⁻¹) and high mean concentrations of dissolved-N (0.65 mg TDN L⁻¹ and 0.13 mg NH₄⁺-N L⁻¹). Stormwater runoff flow volume was low from the 2 ha pasture and contained low sediment and high dissolved nutrient concentrations. Cattle disturbance to canals and nutrient flushing during irrigation were important transport mechanisms for all nutrients. Management considerations for decreasing nutrient export to sensitive waterways in the Wood River Valley include reducing irrigation surface runoff, enhancement of riparian wetlands and vegetation and limiting and excluding livestock access to waterways.
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